Cooling device for MR apparatus

ABSTRACT

The invention relates to a cooling method for cooling a superconducting coil assembly in a MR apparatus, wherein the superconducting coil assembly  10  is cooled using a cooling agent  41, 42  which is in thermal contact with the superconducting coil assembly in a cooling chamber  20 , the cooling agent being cooled by a refrigerator  50 . The method comprises the steps of transferring (S 2 ) cooling agent from the cooling chamber to a cooling agent storage when a predetermined temperature is exceeded in at least a part of the cooling agent in the cooling chamber, and returning (S 4 ) cooling agent from the cooling agent storage to the cooling chamber when the temperature of at least a part of the cooling agent in the cooling chamber is equal to or less than the predetermined temperature. The invention also relates to a cooling device for performing the cooling method and to an MR apparatus with such a cooling device.

This invention relates to a cooling device for cooling a superconductingcoil assembly in an MR apparatus, comprising a cooling chamber adaptedto contain a cooling agent which is in thermal contact with thesuperconducting coil assembly, a refrigerator for cooling the coolingagent and an MR apparatus with a respective cooling device. Theinvention also relates to a cooling method for cooling a superconductingcoil assembly in an MR apparatus, wherein the superconducting coilassembly is cooled using a cooling agent which is in thermal contactwith the superconducting coil assembly in a cooling chamber, the coolingagent being cooled by a refrigerator.

Cooling devices as described above are well known in the art, adescription of which can be taken from U.S. Pat. No. 5,410,286. Suchcooling devices are used for cooling down a superconducting coilassembly in order to achieve a low temperature at which thesuperconducting material of the coil assembly has superconductingproperties. In this state, the coil is able, due to the absence ofelectrical resistance in the cold magnetic coils, to produce andmaintain a strong magnetic field. Since an increase of temperatureimmediately leads to an increase in electrical resistance in the coil,which again induces heat and therefore leads to a further increase oftemperature, it is essential to keep the temperature in the prescribedlow range. Usually, the coils are enclosed in a cooling chamber filledwith a liquid and/or gaseous cooling agent. The coils are preferablyembedded in a liquid bath of the cooling agent, e.g. helium, having atemperature of around 4K at atmospheric pressure; this is appropriatefor the superconducting materials commonly used in MRI magnets. Tomaintain the liquid helium at this cryogenic temperature, a refrigeratoris used to compensate for heat transfer due to non-ideal isolation.Usually, the refrigerator is adapted to provide sufficient cooling powerto allow zero-boil-off operation. In such zero-boil-off operation therefrigerator has a cooling capacity which is sufficient for cooling thehelium in normal use in such a way that the temperature is kept within aprescribed range.

However, in given circumstances, irregularities may occur which canaffect the cooling behavior of the cooling device. As such, the coolingefficiency might be lowered or even completely stopped due to defects,power breakdowns, leakages or other events leading to a partial orcomplete shut down of the refrigerator or to increased heat transferinto the cooling agent.

In times of such irregularities the temperature of at least parts of thecooling agent increases, resulting in an increase of pressure inside thecooling chamber. Consequently it is usually required to release heliumfrom the cooling chamber, being only able to load a certain pressureinside. Consequently, a loss of helium occurs each time when such anirregularity happens. To assure that the coil is always embedded inliquid helium, a refill procedure has to take place at certain timeintervals to compensate for this loss of helium. This refill procedurerequires additional management efforts and leads to down times of the MRdevice. As a result, the efficiency of the device is lowered andoperational costs increase.

Moreover, since the world's helium resources are limited, a loss ofhelium should be avoided.

Thus, it is the object of this invention to provide a cooling devicewith a reduced loss of cooling agent.

This object is achieved according to the invention by a cooling deviceas described in the preamble of claim 1, having a cooling agent storagein fluid connection with the cooling chamber, the storage being adaptedto take up cooling agent from the cooling chamber when at least a partof the cooling agent in the cooling chamber exceeds a firstpredetermined temperature and to return cooling agent to the coolingchamber when at least a part of the cooling agent in the cooling chamberremains below or is equal to a second predetermined temperature.

The use of the proposed cooling agent storage allows storage of thecooling agent in operational conditions, in which it was usually blownoff. A loss of cooling agent is thus avoided. Furthermore, the coolingdevice according to the invention allows immediate return of coolingagent to the cooling chamber as soon as the abnormal operationalconditions have ended and the cooling device returns to normaloperation. In such normal operational conditions the cooling capacity ofthe refrigerator usually allows cooling down of additional coolingagent, which is returned to the cooling chamber from the cooling agentstorage at a temperature higher than that of the cooling agent in thecooling chamber itself.

The cooling device according to the invention provides a closed systemin which the cooling agent can be transferred to the cooling agentstorage in times of abnormal operational conditions which required ablow off of cooling agent in the state of the art. The cooling agent canbe stored in the cooling agent storage at a temperature higher than thetemperature of the cooling agent in the cooling chamber. As soon as thecooling device returns to normal operational conditions or additionalcooling capacity is provided in the system, the cooling agent stored inthe cooling agent storage can be transferred to the cooling chamberagain and be cooled down to the temperature of the cooling agent whichremained in the cooling chamber.

The cooling device is particularly advantageous when used in systemshaving a cooling capacity sufficient to be operated at zero boil-off. Insuch systems, an increase and decrease of the temperature of the coolingagent in at least a part of the cooling chamber will occur due to adifference of the cooling power introduced into the cooling chamber bythe refrigerator and the sum of the heat transfer into the chamber andthe heat induction in the chamber. For example, increased heat transferdue to a defective insulation or decreased cooling power will induce atemperature increase and the liquid cooling agent in the cooling chamberwill turn to gas. This will immediately lead to a transfer of coolingagent to the cooling agent storage. A temperature increase in the wholecooing chamber is thus avoided. Conversely, the cooling agent will beretransferred as soon as the cooling power increases and exceeds the sumof the heat transfer into the cooling chamber and the heat induced inthe cooling chamber, making the gaseous cooling agent in the coolingchamber condense on the cooling surface.

In a preferred embodiment according to the invention the cooling chamberis adapted to contain cooling agent in liquid and a gaseous conditionand the fluid connection is connected to a part of the cooling chamberwhich is adapted to contain a gaseous cooling agent. This embodiment isparticularly advantageous when the superconducting coil assembly isembedded in the liquid cooling agent, ensuring that the superconductingtemperature of the coil material is maintained even in periods ofabnormal operational conditions, such as failure of the refrigerator.Providing the fluid connection in a part of the cooling chambercontaining gaseous cooling agent allows for the transfer of only gaseouscooling agent to the cooling agent storage and avoids the transfer ofcryogenic, liquefied cooling agent. Thus, easy control of the pressureinside the cooling chamber is achieved and excessive loss of coolingagent, in particular liquid cooling agent, is avoided.

According to another preferred embodiment of the invention, means areprovided for controlling the take up and return of the cooling agent bymeans of a signal derived from the pressure of the cooling agent in thecooling chamber.

The cooling agent contained in the cooling chamber according to theinvention has a pressure higher than the surrounding atmospherepressure, as it is well known from state of the art cooling devices. Itis thus prevented that contaminants can be drawn into the coolingchamber from the ambient atmosphere. To maintain this higher pressureinside the cooling chamber, the cooling chamber is sealed from theambient atmosphere. Abnormal working conditions, such as increased heatinduction in the cooling chamber or lowered cooling capacity of therefrigerator, induce an expansion of the gaseous cooling agent and/or atransition of liquefied cooling agent into the gaseous condition. Thisinduces a pressure increase inside the cooling chamber is induced,enabling the extraction of a signal to control take up and return of thecooling agent. Using this signal, a transfer of the cooling agent to thecooling agent storage and a return from the cooling agent storage to thecooling chamber can be easily controlled by the pressure of the coolingagent inside the cooling chamber.

In another preferred embodiment of the invention the refrigeratorcomprises a cooling surface in thermal contact with the cooling agent,the cooling surface extending into the cooling chamber, in particularinto that part of the cooling chamber which is adapted to containgaseous cooling agent. A simple arrangement of the refrigerator coolingthe cooling agent within the cooling chamber is thus achieved. Theembodiment also avoids multiple fittings required for other refrigeratorarrangements in which the cooling agent has to be piped to an externalcooling surface. As far as the cooling surface extends into the part ofthe cooling chamber containing gaseous cooling agent, a simple internalcooling cycle is achieved inside the cooling chamber, in which gaseouscooling agent is cooled down in a region around the cooling surface,thereby condensing on the cooling surface and dropping into the pool ofliquid cooling agent.

In another preferred embodiment the cooling agent storage includes agasometer for storing the cooling agent at a constant predeterminedpressure. The use of a gasometer for storing the cooling agent allowssimple and safe storage. A risk of explosion of the storage, as alwaysexists when highly compressed medium is stored in a closed storage ofconstant volume, is avoided, since the gasometer is able to increase itsstorage volume according to the volume of the cooling agent introducedinto the gasometer. Furthermore, in a similar fashion as in a storage ofconstant volume, cooling agent can be released from the storage when acertain amount of cooling agent inside the storage is exceeded.

According to another preferred embodiment of the invention the coolingagent storage comprises a pressure tank in fluid connection with thecooling chamber for taking up the compressed cooling agent, a compressormeans interposed in a fluid connection between the cooling chamber andthe pressure tank for compressing the cooling agent exiting the coolingchamber, and a pressure reduction means interposed in a fluid connectionbetween the cooling chamber, and the pressure tank for reducing thepressure of the cooling agent returning to the cooling chamber. Aspace-saving cooling device is thus realized since the compression ofthe cooling agent and its storage in a pressure tank allow the storageof a large mass of cooling agent in a rather small space. The coolingagent can be compressed so far, that it reaches a liquid condition andcan be stored in this liquid condition, so that a large mass of coolingagent is stored in a smaller space in comparison with the storage in agasometer.

Since the cooling agent in the cooling chamber is usually at a pressurewhich is only slightly above the atmospheric pressure and the compressormeans compress the cooling agent to a pressure well above the pressureof the cooling agent in the cooling chamber, it is necessary to reducethe pressure of the cooling agent before it is retransferred to thecooling chamber. The pressure reduction means for achieving thispressure reduction could be, for example, a valve or throttle.

In another preferred embodiment of the invention the cooling agentstorage is adapted to contain the cooling agent in a gaseous condition.This embodiment is preferred when a safe and cost-effective storage isneeded. Storing the cooling agent in a gaseous condition allows storageat atmospheric pressure or slightly above this pressure and at roomtemperature or temperatures below but close to room temperature.

Finally, in a last preferred embodiment the cooling chamber and thecooling agent storage are adapted to contain helium as the coolingagent. Helium is particularly useful for use as cooling agent, sincehelium has a temperature of around 4K (4° above absolute zero) in theliquid/gaseous condition at atmospheric pressure (approx. 1013 mbar) orslightly above atmospheric pressure. This temperature is sufficient tocool a variety of superconducting materials to a temperature at whichthey have superconducting properties.

Another aspect of the invention is a cooling method for cooling asuperconducting coil assembly in a MR apparatus, wherein thesuperconducting coil assembly is cooled using a cooling agent which isin thermal contact with the superconducting coil assembly in a coolingchamber, the cooling agent being cooled by a refrigerator, the methodcomprising the steps of transferring cooling agent from the coolingchamber to a cooling agent storage when a predetermined temperature isexceeded in at least a part of the cooling agent in the cooling chamberand returning cooling agent from the cooling agent storage to thecooling chamber when the temperature of at least a part of the coolingagent in the cooling chamber is equal to or less than the predeterminedtemperature.

The cooling method according to the invention allows safe cooling of asuperconducting coil assembly without loss of cooling agent in times ofabnormal operational conditions. The cooling agent is kept within aclosed system. The cooling method according to the invention can easilybe performed by means of known cooling devices when they areadditionally equipped with a cooling agent storage as described in thecharacterizing part of claim 1. This allows an effective way of coolingsuperconducting magnets in existing MR apparatus.

The cooling method according to the invention can be further improvedwhen the cooling agent is in a gaseous and a liquid condition in thecooling chamber and the transfer and return of the cooling agent in thegaseous condition is controlled by means of a signal derived from thepressure of the cooling agent inside the cooling chamber and the coolingagent is transferred from the cooling chamber to the cooling agentstorage when a first predetermined pressure is exceeded in the coolingchamber and the cooling agent is returned from the cooling agent storageto the cooling chamber when the pressure of the cooling agent in thecooling chamber is equal to or less than a second predeterminedpressure.

This embodiment of the cooling method is particularly advantageous sinceusually the cooling agent is contained in a closed cooling chamber at afirst pressure above but close to atmospheric pressure. Even a smalltemperature increase in parts of the cooling agent then leads to apressure increase inside the closed cooling chamber. This pressureincrease allows for easy detection of a partially temperature increase.As soon as the respective parts of the cooling agent are cooled to thedesired, predetermined temperature or below this temperature, thepressure inside the cooling chamber returns to the respective secondpredetermined pressure or drops below this pressure. In this situation,cooling agent can be retransferred to the cooling chamber so as tocompensate the aforementioned loss of cooling agent. The first and thesecond predetermined pressure may lie at the same pressure level.

In another preferred embodiment of the cooling method according to theinvention the transferred cooling agent is compressed so as to be storedin a compressed state outside the cooling chamber and decompressed so asto be returned to the cooling chamber. This embodiment allows forspace-saving storage of the cooling agent, since a large mass of coolingagent can be stored in a small space when it is compressed prior tobeing introduced into the storage. Since the cooling chambers of most MRapparatuses are arranged to operate at pressures close to atmosphericpressure, decompression of the cooling agent is required before it canbe retransferred to the cooling chamber. Compression could be achievedby a fan or blower. To achieve higher compression rates, a compressor oreven a condenser can be used. Decompression could be achieved by a valveor throttle and has to be performed in a way that it is adapted to therate of compression inside the storage in relation to the pressureinside the cooling chamber. It has to be assured that the decompressionis performed in a way that a predetermined pressure, usually being closeto atmospheric pressure, inside the cooling chamber is not exceeded whenthe cooling agent is returned.

Another aspect of the invention is an MR apparatus, comprising asuperconducting magnet having a superconducting coil assembly and acooling device as described above for cooling said superconducting coilassembly.

Preferred embodiments of the invention will now be explained withreference to the accompanying figures, wherein

FIG. 1 is a schematic representation of a first embodiment according tothe invention,

FIG. 2 is a schematic representation of a second embodiment according tothe invention,

FIG. 3 is a flow chart of a first preferred embodiment of the coolingmethod according to the invention, and

FIG. 4 is a flow chart of a second preferred embodiment of the coolingmethod according to the invention.

Referring to FIG. 1, a first embodiment of the invention comprises an MRimaging device having a superconducting coil assembly 10 arranged insidea cylindrical cooling chamber 20. The coil assembly 10 and the coolingchamber 20 are shown in a cross-sectional view in FIG. 1. Thecylindrical cooling chamber 20 surrounds a cylindrical examination spacearranged to accommodate a person to be examined with the aid of the MRIdevice.

The cylindrical cooling chamber 20 comprises a dome 21 disposed at theupper side of the cooling chamber 20. The cooling chamber 20 is filledwith helium in liquid (41) and gaseous (42) condition. The amount of thehelium in liquid condition 41 is such that the coil assembly 10 iscompletely immersed in the liquid helium 41. The lower part 22 of thecylindrical cooling chamber 20 is completely filled with liquid helium,whereas in the upper part 23 of the cooling chamber 20 a certain levelof liquid helium is reached and above this level gaseous helium 42 ispresent.

The dome 21 is arranged in such a way that gaseous cooling agent iscollected therein. Due to well-known physical effects and properties offluids like this gaseous helium, in particular that amount of gaseoushelium is collected in the dome which has a temperature lying above thetemperature of the liquid helium and the gaseous helium in the upperpart 23 of the cooling chamber 20.

A refrigerator 50 is arranged in the vicinity of the cooling chamber 20.The refrigerator 50 comprises a cooling surface 51 extending into thedome 21 of the cooling chamber 20. The temperature of the coolingsurface 51 is controlled in such a way that it lies below thetemperature which is required in the cooling agent to achieve thesuperconducting properties of the coil assembly 10. For example, when aliquid helium temperature of approximately 4K is sufficient forachieving superconducting properties of the coil assembly 10, thetemperature of the cooling surface 51 might be 3.8K However, atemperature of 4.2 to 4K of the liquid helium 41 is sufficient for mostsuperconducting materials. Gaseous cooling agent above approx. 4.2Kcondenses on the cooling surface 51 and drops back into the pool ofliquid helium 41 due to gravity.

In the upper part of the dome 21 a first gas conduit 60 is attached byway of a first end fitting which opens into the cooling chamber 20. Thesecond end fitting of the gas conduit 60 is connected to a gasometer 70.In the gasometer 70 helium is stored at a pressure of approximately 300mbar above atmospheric pressure.

The arrangement according to FIG. 1 provides for automatic transfer andretransfer of gaseous helium between the cooling chamber 20 and thegasometer 70 through the gas conduit 60. The transfer and the retransferare automatically controlled by the pressure in the cooling chamberwhich is kept constant at approximately 300 mpa above atmosphericpressure. In times the cooling capacity of the refrigerator goes beyondthe heat transfer into the cooling chamber, an immediate transfer ofcooling agent to the gasometer is thus achieved by way of a leveling outof the pressure inside the cooling chamber and the pressure inside thegasometer.

FIG. 2 shows a second embodiment of the cooling device according to theinvention. The embodiment of FIG. 2 is similar to that of FIG. 1 inrespect of the coil assembly 10, the cooling chamber 20, therefrigerator 50 and the cooling agent 41, 42 inside the cooling chamber20. Identical parts of FIGS. 1 and 2 are labeled with identicalreference numerals, a detailed description of which is omitted below.

As opposed to the embodiment of FIG. 1, the embodiment of FIG. 2 isprovided with two gas conduits 60 a′, a″, b′, b″. A first part of thefirst gas conduit 60 a′ is connected to the upper part of the dome 21and is arranged for transferring gaseous helium to a compressor 80. Thecompressor 80 is designed to compress the gaseous helium to a pressureof approximately 100 bar. The compressed helium is stored in acompression-proof storage 90. For this purpose, a second part of thefirst gas conduit 60 a″ connects the pressure side of the compressor 80to the helium gas storage 90.

A second gas conduit 60 b′, b″ serves for retransferring helium gas fromthe helium gas storage 90 to the cooling chamber 20. In particular, thehelium gas storage 90 is connected to a pressure regulator 100 via afirst part of a second gas conduit 60 b′.

The pressure regulator 100 throttles the helium gas to a pressure ofapproximately 300 mbar above atmospheric pressure. The pressureregulator 100 is connected to the upper part of the dome 21 via a secondpart of the second gas conduit 60 b″. Throttled helium gas can thus beretransferred from the helium gas storage 90 to the cooling chamber 20via the pressure regulator 100.

The helium gas thus returned will usually have a temperature above thetemperature of the helium gas inside the cooling chamber. Since thereturned helium gas is introduced into the dome close to the coolingsurface 51 of the refrigerator, this gas is immediately cooled down andcondenses on the cooling surface 51, thereby reaching the temperaturerequired for cooling the superconducting coil assembly 10.

The pressure regulator 100 is adapted to open and close the second gasconduit 60 b′, b″ in dependence on the gas pressure in the second partof the second gas conduit 60 b″ which corresponds to the gas pressureinside the dome 21.

Furthermore, the pressure regulator 100 is adapted to activate ordeactivate the compressor 80 in dependence on this pressure signal. Forexample, when a certain pressure, for example 320 mbar above atmosphericpressure, is exceeded in the second part of the second gas conduit 60b″, or the dome 21, the compressor 80 is activated. As soon as thepressure drops below a second predetermined value, for example 300 mbarabove atmospheric pressure, the compressor 80 is deactivated. As soon asthe pressure drops below a third predetermined value, for example 280mbar above atmospheric pressure, the pressure regulator opens the secondgas conduit 60 b, thereby retransferring helium gas to the coolingchamber.

It should be noted that the pressure in the dome 21 corresponds to thepressure in the first part of the first gas conduit 60 a′ as well and,therefore, the pressure signal could alternatively be taken from thispart.

Referring now to FIG. 3, in a preferred embodiment of the cooling methodaccording to the invention a cooling agent is contained in a coolingchamber in a first step S1. When this helium cooling agent exceeds apressure difference of 320 mbar, for example, above the atmosphericpressure in decision D1, a part of said helium is transferred to ahelium storage in a step S2. In this helium storage it is stored in astep S3 until it is determined in D2 that, for example, the pressuredifference between the cooling chamber and the atmosphere is below orequal to 280 mbar. As soon as this condition is fulfilled, a part of thehelium stored in the storage is returned to the cooling chamber in astep S4.

Alternatively, the cooling chamber and the helium storage can beconnected by way of a respective pipe, allowing an exchange of gaseouscooling agent and keeping the pressure in the cooling chamber and thehelium storage at equal levels at all times. The transfer and theretransfer of the helium can thus be easily performed, without anycontrol means, simply by way leveling out the pressure inside theconnected volumes of the cooling chamber and the helium storage.

Referring now to FIG. 4, showing a second preferred embodiment of thecooling method according to the invention, in similar steps S1 anddecision D1 the cooling agent is stored and a decision is made independence on the pressure difference between the cooling chamber andthe atmosphere. If the condition of D1 is fulfilled, the cooling agentis transferred to a compressor in a step S2 a Afterwards, the coolingagent is compressed to a maximum pressure of 100 bar above atmosphericpressure in a step S2 b and transferred to the cooling agent storage ina step S2 c.

Similar to the embodiment of FIG. 3, the cooling agent is stored in thestorage in a step S3 and depending on a decision D2 which is similar toFIG. 3, it is transferred to a throttle in a step S4 a. In thisthrottle, the cooling agent is decompressed to a pressure of 300 mbarabove atmospheric pressure in a step S4 b and afterwards returned to thecooling chamber in a step S4 c.

The invention provides a cooling device for MR apparatus, in particularfor apparatus adapted for zero boil-off operation, which is arranged toavoid any release of cooling agent, even if the cooling power of therefrigerator of the cooling device decreases and the heat transfer intothe cooling chamber lasts for a longer time period. Costs due to theneed for refilling cooling agent can thus be saved. The limitedresources of commonly used cooling agents, such as helium, are notwasted. The method according to the invention allows for efficient useof cooling agents and can be applied in known MR apparatus as well as innewly designed MR apparatus, without necessitating the addition ofexpensive equipment.

1. A cooling device for cooling a superconducting coil assembly in an MRapparatus, comprising: a cooling chamber adapted to contain a coolingagent which is in thermal contact with the superconducting coilassembly, a refrigerator for cooling the cooling agent, and a coolingagent storage in fluid connection with the cooling chamber, the coolingagent storage being adapted to take up cooling agent from the coolingchamber when at least a part of the cooling agent in the cooling chamberexceeds a first predetermined temperature, and to return cooling agentto the cooling chamber when at least a part of the cooling agent in thecooling chamber remains below or is equal to a second predeterminedtemperature, the cooling agent storage including a pressure tank influid connection with the cooling chamber for taking up the compressedcooling agent and a compressor means interposed in a fluid connectionbetween the cooling chamber and the pressure tank in order to compressthe cooling agent exiting the cooling chamber.
 2. A cooling deviceaccording to claim 1, wherein the cooling chamber is adapted to containa cooling agent in a liquid and a gaseous condition and the fluidconnection is connected to a part of the cooling chamber which isadapted to contain a gaseous cooling agent.
 3. A cooling deviceaccording to claim 1, wherein the refrigerator has a cooling powersufficient to compensate heat transfer to the cooling chamber in regularcondition so as to allow zero boil-off operation.
 4. A cooling deviceaccording to claim 1, comprising means for controlling the take up andreturn of the cooling agent by means of a signal derived from thepressure of the cooling agent in the cooling chamber.
 5. A coolingdevice according to claim 1, wherein the refrigerator comprises acooling surface in thermal contact with the cooling agent, the coolingsurface extending into the cooling chamber, in particular into that partof the cooling chamber which is adapted to contain a gaseous coolingagent.
 6. A cooling device according to claim 1, wherein the coolingagent storage includes a gasometer for storing the cooling agent at aconstant predetermined pressure.
 7. A cooling device according to claim1, wherein the cooling agent storage comprises a pressure reductionmeans interposed in a fluid connection between the cooling chamber andthe pressure tank in order to reduce the pressure of the cooling agentreturning to the cooling chamber.
 8. A cooling device according to claim1, wherein the cooling agent storage is adapted to contain the coolingagent in a gaseous condition.
 9. A cooling device according to claim 1,wherein the cooling chamber and the cooling agent storage are adapted tocontain helium as a cooling agent.
 10. A cooling method for cooling asuperconducting coil assembly in an MR apparatus, wherein thesuperconducting coil assembly is cooled using a cooling agent which isin thermal contact with the superconducting coil assembly in a coolingchamber, the cooling agent being cooled by a refrigerator, the methodcomprising the steps of: transferring cooling agent from the coolingchamber to a cooling agent storage when a predetermined temperature isexceeded in at least a part of the cooling agent in the cooling chamber,and returning cooling agent from the cooling agent storage to thecooling chamber when the temperature of at least a part of the coolingagent in the cooling chamber is equal to or less than the predeterminedtemperature, wherein the transferred cooling agent is compressed so asto be stored in a compressed state outside the cooling chamber anddecompressed so as to be returned to the cooling chamber.
 11. A coolingmethod according to claim 10, wherein the cooling agent is in a gaseousand a liquid condition in the cooling chamber and the transfer andreturn of the cooling agent in the gaseous condition is controlled bymeans of a signal derived from the pressure of the cooling agent insidethe cooling chamber, and the cooling agent is transferred from thecooling chamber to the cooling agent storage when a first predeterminedpressure is exceeded in the cooling chamber, and the cooling agent isreturned from the cooling agent storage to the cooling chamber when thepressure of the cooling agent in the cooling chamber is equal to or lessthan a second predetermined pressure.
 12. An MR apparatus comprising: asuperconducting magnet having a superconducting coil assembly; and acooling device for cooling said superconducting coil assembly, thecooling device comprising: a cooling chamber adapted to contain acooling agent which is in thermal contact with the superconducting coilassembly, a refrigerator for cooling the cooling agent, a pressure tankin fluid connection with the cooling chamber for taking up thecompressed cooling agent and returning cooling agent to the coolingchamber in accordance with temperature in the cooling chamber, and acompressor interposed in a fluid connection between the cooling chamberand the pressure tank in order to compress the cooling agent exiting thecooling chamber.
 13. An MR apparatus according to claim 12, furtherincluding a pressure reducer interposed in a fluid connection betweenthe cooling chamber and the pressure tank in order to reduce thepressure of the cooling agent returning to the cooling chamber.